And as soon as the mountains were built, they began, just as ineluctably, to wear
away. For all their seeming permanence, mountains are exceedingly transitory features.
In Meditations at 10,000 Feet, writer and geologist James Trefil calculates that a typical
mountain stream will carry away about 1,000 cubic feet of mountain in a year, mostly in
the form of sand granules and other suspended particles. That is equivalent to the
capacity of an average-sized dump truck--clearly not much at all. Imagine a dump truck
arriving once each year at the base of a mountain, filling up with a single load, and driving
off, not to reappear for another twelve months. At such a rate it seems impossible that it
could ever cart away a mountain, but in fact given sufficient time that is precisely what
would happen. Assuming a mountain 5,000 feet high with 500,000 million cubic feet of
mass--roughly the size of Mount Washington--a single stream would level it in about 500
million years.
Of course most mountains have several streams and moreover are exposed to a vast
range of other reductive factors, from the infinitesimal acidic secretions of lichen (tiny but
relentless!) to the grinding scrape of ice sheets, so most mountains vanish very much
more quickly--in a couple of hundred million years, say. Right now the Appalachians are
shrinking on average by 0.03 millimeters per year. They have gone through this cycle at
least twice, possibly more--rising to awesome heights, eroding away to nothingness, rising
again, each time recycling their component materials in a dazzlingly confused and
complex geology.
The detail of all this is theory, you understand. Very little of it is more than generally
agreed upon. Some scientists believe the Appalachians experienced a fourth, earlier
mountain-building episode, called the Grenville Orogeny, and that there may have been
others earlier still. Likewise, Pangaea may have split and reformed not three times but a
dozen times, or perhaps a score of times. On top of all this, there are a number of lapses
in the theory, chief of which is that there is little direct evidence of continental collisions,
which is odd, even inexplicable, if you accept that at least three continents rubbed
together with enormous force for a period of at least 150 million years. There ought to be
a suture, a layer of scar tissue, stretching up the eastern seaboard of the United States.
There isn't.
I am no geologist. Show me an unusual piece of grey wacke or a handsome chunk of
gabbro and I will regard it with respect and listen politely to what you have to say, but it
won't actually mean anything to me. If you tell me that once it was seafloor ooze and that
through some incredible sustained process it was thrust deep into the earth, baked and
squeezed for millions of years, then popped back to the surface, which is what accounts
for its magnificent striations, its shiny vitreous crystals, and flaky biotate mica, I will say,
"Goodness!" and "Is that a fact!" but I can't pretend that anything actual will be going on
behind my game expression.
Just occasionally am I permitted an appreciative glimpse into the wonder that is
geology, and such a place is the Delaware Water Gap. There, above the serene Delaware
River, stands Kittatinny Mountain, a wall of rock 1,300 feet high, consisting of resistant
quartzite (or so it says here) that was exposed when the river cut a passage through
softer rock on its quiet, steady progress to the sea. The result in effect is a cross-section
of mountain, which is not a view you get every day, or indeed anywhere else along the
Appalachian Trail that I am aware of. And here it is particularly impressive because the
sean pound
(Sean Pound)
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